As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having a high energy density and discharge voltage, into which much research has been carried out and which is now commercialized and widely used.
In general, a secondary battery is configured to have a structure in which an electrode assembly including a positive electrode and a negative electrode stacked or wound in a state in which a separator is disposed between the positive electrode and the negative electrode is mounted in a battery case made of a metal can or a laminate sheet, and an electrolyte is injected into the battery case or the electrode assembly is impregnated with the electrolyte.
The electrolyte in the secondary battery is decomposed during repeated charge and discharge of the secondary battery. As a result, gas is generated from the secondary battery. Particularly for a pouch-shaped secondary battery, a battery cell mounted in the secondary battery is sealed in a vacuum state so as to minimize contact between the battery cell and external air. For this reason, it is substantially impossible to discharge the generated gas out of the battery cell and to further inject an electrolyte into the battery cell.
Therefore, there is a high necessity for a novel lithium secondary battery that is capable of solving the above problems.